Recyclable, self-strengthening starch-based epoxy vitrimer facilitated by exchangeable disulfide bonds

Epoxy vitrimers have emerged as a new class of self-healing, recyclable, and reprocessable materials, offering new opportunities to traditional epoxy thermosets by improving life-span, while providing additional functionalities. Nevertheless, retaining 100 % of the original mechanical performances remains difficult for vitrimers after several reprocessing cycles due to progressive changes in the vitrimer networks during rearrangements. In this study, we designed a novel epoxy vitrimer with a higher renewable content compared to conventional epoxies by using renewable materials. The bio-based epoxy vitrimer was synthesized from epoxidized starch amylopectin together with diallyl disulfide, that is naturally found in garlic, and a thiol (pentaerythritol tetrakis(3-mercaptopropionate) (PETMP)). Diallyl disulfide and PETMP enabled the formation of a recyclable, and reprocesseable, vitrimer network. The epoxy vitrimer displayed unprecedented self-strengthening after 5 recycling cycles (tensile strength increased over 900 %) caused by the mechanically-induced homogeneization of the diallyl disulfide/thiol and the starch epoxy ghost granule phases during the recycling process, thereby increasing the vitrimer cross-linking density during reformation. Reprocessing the vitrimer 5-times improved the mechanical and thermal properties, raising glass transition temperature, Young's modulus, and tensile strength from 7 °C to 25 °C, 2.98 MPa to 268 MPa, and 1.87 MPa to 18.47 MPa, respectively. Hence, capitalizing on mechanically-induced phase homegeneization during the vitrimer reprocessing, this work introduces a strategy for the design of self-strengthening bio-based and recyclable thermosets.